Process for removing sulfur oxides from a gas

ABSTRACT

Sulfur oxides are removed from a gas by an absorbent which comprises an inorganic oxide composition in association with at least one free or combined rare earth metal selected from the group consisting of lanthanum, cerium, praseodymium, samarium and dysprosium, wherein said inorganic oxide composition is selected from the group consisting of MgAl 2  O 4 , mixtures of alumina with magnesium oxide and mixtures of magnesium oxide with MgAl 2  O 4 . The absorbent can be circulated through a fluidized catalytic cracking process together with hydrocarbon cracking catalyst to reduce sulfur oxide emissions from the regeneration zone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 29,264, filedApr. 11, 1979 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for reducing the sulfur oxidecontent of a waste gas stream through the use of absorbents which can bereactivated for further absorption of sulfur oxides by contact with ahydrocarbon in the presence of a hydrocarbon cracking catalyst. Moreparticularly, this invention relates to a method for reducing sulfuroxide emissions from the regenerator of a fluidized catalytic crackingunit.

2. Description of the Prior Art

A major industrial problem involves the development of efficient methodsfor reducing the concentration of air pollutants, such as sulfur oxides,in the waste gases which result from the processing and combustion ofsulfur containing fuels. The discharge of these waste gas streams intothe atmosphere is environmentally undesirable at the sulfur oxideconcentrations which are frequently encountered in conventionaloperations. Such waste gas streams typically result, for example, fromoperations such as the combustion of sulfur containing fossil fuels forthe generation of heat and power, the regeneration of catalysts employedin the refining of hydrocarbon feedstocks which contain organic sulfurcompounds, and the operation of Claus-type sulfur recovery units.

Two fundamental approaches have been suggested for the removal of sulfuroxides from a waste gas. One approach involves scrubbing the waste gaswith an inexpensive alkaline material, such as lime or limestone, whichreacts chemically with the sulfur oxides to give a nonvolatile productwhich is discarded. Unfortunately, this approach requires a large andcontinual supply of the alkaline scrubbing material, and the resultingreaction products can create a solid waste disposal problem ofsubstantial magnitude.

The second principal approach to the control of sulfur oxide emissionsinvolves the use of sulfur oxide absorbents which can be regeneratedeither thermally or chemically. The process of the subject invention isrepresentative of this second approach.

U.S. Pat. No. 4,001,375 to J. M. Longo discloses a process for removalof sulfur oxides from a gas which involves absorbing the sulfur oxideswith cerium oxide followed by regeneration of the spent cerium oxide byreaction with hydrogen gas. This regeneration step results in theformation of a gas which contains a 1:1 ratio of hydrogen sulfide tosulfur dioxide and which may be fed directly to a Claus-type sulfurrecovery unit for conversion into elemental sulfur. It is furtherdisclosed that the cerium oxide may be supported on an inert supportsuch as alumina, silica and magnesia. The patent does not, however,suggest that the spent cerium oxide could be regenerated by contact witha hydrocarbon in the presence of a hydrocarbon cracking catalyst. Inaddition, the patent fails to suggest that the nature of the supportcould be significant or that cerium can be combined with alumina and/ormagnesia to effect an absorption of sulfur oxides which is enhanced as aconsequence of synergism.

An article entitled "Selection of Metal Oxides for Removing SO₂ fromFlue Gas" by Lowell et al. in Ind. Eng. Chem. Process Des. Develop.,Vol. 10, No. 3, 1971, is addressed to a theoretical evaluation of thepossible use of various metal oxides to absorb sulfur dioxide from aflue gas. The authors evaluate 47 metal oxides from which they select agroup of 16 potentially useful single oxide absorbents, which includesthe oxides of aluminum, cerium and titanium. Magnesium oxide waseliminated from the group of potentially useful oxides because of anunfavorable sulfate decomposition temperature. This evaluation is basedon the assumption that the absorbents would be regenerated thermally anddoes not consider the possibility of regeneration under reducingconditions. Consequently, there is no suggestion that any of these metaloxides could be regenerated by contact with a hydrocarbon in thepresence of a hydrocarbon cracking catalyst.

The cyclic, fluidized, catalytic cracking of heavy petroleum fractionsis one of the major refining operations involved in the conversion ofcrude petroleum oils to valuable products such as the fuels utilized ininternal combustion engines. Such a process involves the cracking of apetroleum feedstock in a reaction zone through contact with fluidizedsolid particles of a cracking catalyst. Catalyst which is substantiallydeactivated by nonvolatile coke deposits is then separated from thereaction zone effluent and stripped of volatile deposits in a strippingzone. The stripped catalyst particles are separated from the strippingzone effluent, regenerated in a regeneration zone by combustion of thecoke with an oxygen containing gas, and the regeneration catalystparticles are returned to the reaction zone. In the application of thisprocess to sulfur-containing feedstocks, catalyst is deactivated throughthe formation of sulfur-containing deposts of coke. In conventionalprocesses, the combustion of this sulfur-containing coke results in therelease of substantial amounts of sulfur oxides to the atmosphere. U.S.Pat. No. 3,835,031, to R. J. Bertolacini et al. discloses a method forthe reduction of these sulfur oxide emissions through the use of acracking catalyst comprising a zeolite in a silica-alumina matrix whichhas from about 0.25 to about 5.0 weight percent of a Group IIA metal ormixture of Group IIA metals distributed over the surface of the matrixand present as an oxide or oxides. The metal oxide or oxides react withsulfur oxides in the regeneration zone to form nonvolatile inorganicsulfur compounds. These nonvolatile inorganic sulfur compounds are thenconverted to the metal oxide or oxides and hydrogen sulfide uponexposure to hydrocarbons and steam in the reaction and stripping zonesof the process unit. The resulting hydrogen sulfide is disposed of inequipment conventionally associated with a fluid catalytic crackingunit. Similarly, Belgian Pat. No. 849,637 also is directed to a processwherein a Group IIA metal or metals is circulated through a cyclicfluidized catalytic cracking process in order to reduce the sulfur oxideemissions resulting from regeneration of deactivated catalyst. Thedisclosures of these patents do not, however, suggest the desirabilityof combining a rare earth metal with the oxide of a Group IIA metal suchas magnesium oxide or calcium oxide.

Belgian Pat. No. 849,636 and its counterpart, U.S. pat. application Ser.No. 748,556, disclose a process similar to that set forth in U.S. Pat.No. 3,835,031, which involves the removal of sulfur oxides from theregeneration zone flue gas of a cyclic, fluidized, catalytic crackingunit through the use of a zeolite-type cracking catalyst in combinationwith a regenerable metallic reactant which absorbs sulfur oxides in theregeneration zone and releases the absorbed sulfur oxides as hydrogensulfide in the reaction and stripping zones of the process unit. It istaught that a suitable metallic reactant comprises one or more membersselected from the group consisting of sodium, scandium, titanium,chromium, molybdenum, manganese, cobalt, nickel, antimony, copper, zinc,cadmium, the rare earth metals and lead, in free or combined form. Inaddition, it is disclosed that the metallic reactant may be supported byan amorphous cracking catalyst or a solid which is substantially inertto the cracking reaction. Silica, alumina and mixtures of silica andalumina are mentioned as suitable supports. There is no specificteaching, however, of the desirability of combining any particular rareearth metals with inorganic oxides selected from the group consisting ofthe oxides of aluminum, magnesium, zinc, titanium and calcium. Thedisclosure contains no suggestion that such a combination would afford asynergistically enhanced reduction of regenerator sulfur oxideemissions.

Belgian Pat. No. 849,635 and its counterpart, U.S. patent applicationSer. No. 748,555 are also directed to a process of the type set forth inU.S. Pat. No. 3,835,031 and Belgian Pat. No. 849,636, and teaches thatan improved reduction of regeneration zone sulfur oxide emissions can beachieved by combining a sulfur oxide absorbent with a metallic promoter.The metallic promoter comprises at least one free or combined elementselected from the group consisting of ruthenium, rhodium, palladium,osmium, iridium, platinum, vanadium, tungsten, uranium, zirconium,rhenium and silver. The sulfur oxide absorbent comprises at least onefree or combined element which is selected from the group consisting ofsodium, magnesium, calcium, strontium, barium, scandium, titanium,chromium, molybdenum, manganese, cobalt, nickels, antimony, copper,zinc, cadmium, lead and the rare earth metals. Although the metallicpromoter enhances the ability of the absorbent to absorb sulfur oxidesin the regeneration zone of a cyclic, fluidized, catalytic crackingunit, the more active promoters such as platinum and palladium alsopromote the formation of nitrogen oxides and the combustion of carbonmonoxide in the regeneration zone. Since the discharge of nitrogenoxides into the atmosphere is environmentally undesirable, the use ofthese promoters has the effect of substituting one form of undesirableemission for another. The ability of these promoters to enhance thecombustion of carbon monoxide in the regenerator is also undesirable inthose situations wherein the regenerator vessel and associatedequipment, such as cyclones and flue gas lines, are constructed ofmetals such as carbon steel which may not be able to tolerate theincreased regeneration temperatures which can result from enhancedcarbon monoxide combustion.

U.S. Pat. No. 4,146,463 to H. D. Radford et al. discloses a processwherein a waste gas containing sulfur oxides and/or carbon monoxide isconveyed to the regeneration zone of a cyclic, fluidized, catalyticcracking unit wherein it is contacted with a metal oxide which reactswith the sulfur oxides to form nonvolatile inorganic sulfur compounds.This patent teaches that suitable metal oxides include those selectedfrom the group consisting of the oxides of sodium, the Group IIA metals,scandium, titanium, chromium, iron, molybdenum, manganese, cobalt,nickel, antimony, copper, zinc, cadmium, lead and the rare earth metals.In addition, the patent teaches that the metal oxide may be incorporatedinto or deposited onto a suitable support such as silica, alumina andmixtures of silica and alumina. The teachings of this patent fails tosuggest the combination of specific rare earth metals with one or moreinorganic oxides selected from the group consisting of the oxides ofaluminum, magnesium, zinc, titanium and calcium. In addition, there isno suggestion that such a combination could produce improved results asa consequence of synergism.

U.S. Pat. No. 4,071,436 to W. A. Blanton et al. teaches that aluminaand/or magnesia can be used to absorb sulfur oxides from a gas and theabsorbed sulfur oxids can be removed by treatment with a hydrocarbon. Itis further disclosed that sulfer oxide emissions from the regenerator ofa cyclic, fluidized, catalytic cracking unit can be reduced by combiningalumina and/or magnesia with the hydrocarbon cracking catalyst.Similarly, U.S. Pat. Nos. 4,115,249 (W. A. Blanton et al.), 4,115,250(R. L. Flanders et al.) and 4,115,251 (R. L. Flanders et al.) teach theutility of alumina or aluminum to absorb sulfur oxides in theregenerator of a cyclic, fluidized, catalytic cracking unit. Thedisclosures of these patents do not, however, mention the rare earthmetals or suggest that the combination of specific rare earth metalswith alumina and/or magnesia could give improved results.

U.S. Pat. No. 3,899,444 R. E. Stephens is directed to the preparation ofa catalyst support which consists of an inert substrate or core which iscoated with an alumina containing from about 1 to about 45 weightpercent, based on the alumina, of a rare earth metal oxide which isuniformly distributed throughout the alumina coating. It is disclosedthat the inert substrate may include such refractory materials aszirconia, zinc oxide, alumina-magnesia, calcium aluminate, synthetic andnatural zeolites among many others. Similarly, U.S. Pat. No. 4,062,810to W. Vogt et al. discloses compositions comprising cerium oxide on analuminum oxide support.

U.S. Pat. No. 3,823,092 to E. M. Gladrow describes the treatment of azeolite-type hydrocarbon cracking catalyst with a dilute solutioncontaining cerium cations or a mixture of rare earth cations having asubstantial amount of cerium in order to improve the regeneration rateof the catalyst. The resulting catalyst contains between about 0.5 and4.0 percent of cerium oxide and it is further disclosed that thecatalyst matrix may contain from 5 to 30% alumina. Similarly, U.S. Pat.No. 3,930,987 H. S. Grand describes a hydrocarbon cracking catalystcomprising a composite of a crystalline aluminosilicate carrying rareearth metal cations dispersed in an inorganic oxide matrix wherein atleast 50 weight percent of the inorganic oxide is silica and/or alumina,and the rare earth metal content of the matrix if from 1 to 6 percentexpressed as RE₂ O₃. Also, U.S. Pat. No. 4,137,151 to S. M. Csicserydiscloses a composition comprising lanthanum or a lanthanum compound inassociation with a porous inorganic oxide which may be the matrix of azeolite-type cracking catalyst. These patents contain no mention ofsulfur oxides and fail to suggest that the combination of specific rareearth metals with specific metal oxides such as alumina, could afford animproved sulfur oxide absorbent which can be regenerated by contact witha hydrocarbon in the presence of a hydrocarbon cracking catalyst.

Alumina is a component of many different catalyst compositions whichhave been developed for use in the cracking of hydrocarbons. Asynthetically prepared amorphous cracking catalyst, which received widecommercial use shortly after the development of fluidized bed crackingtechniques, contained about 13% alumina and 87% silica. Subsequently,amorphous silica-alumina catalysts were developed and used commerciallywhich contained about 25 to 30% alumina. In addition, silica-magnesiacatalysts were also developed and used commercially. Thesesilica-magnesia catalysts contained about 20% magnesia in addition toabout 15% alumina and about 65% silica. At the present time, most if notall commercial cracking catalysts contain a crystalline aluminosilicateor zeolite which is distributed throughout an amorphous silica-aluminamatrix.

SUMMARY OF THE INVENTION

This invention is directed to a process for removing sulfur oxides froma gas which comprises: (a) absorbing sulfur oxides from the gas with anaborbent which comprises at least one inorganic oxide selected from thegroup consisting of the oxides of aluminum, magnesium, zinc, titaniumand calcium in association with at least one free or combined rare earthmetal selected from the group consisting of lanthanum, cerium,praseodymium, samarium and dysprosium, at a temperature in the rangefrom about 100° to about 900° C., wherein the ratio by weight ofinorganic oxide or oxides to rare earth metal or metals is from about1.0 to about 1,000, and (b) removing said absorbed sulfur oxides fromthe absorbent as a sulfur-contacting said absorbent with a hydrocarbonin the presence of a hydrocarbon cracking catalyst at a temperature inthe range from about 375° to about 900° C.

Another embodiment of the invention is a process for the cyclic,fluidized, catalytic cracking of a hydrocarbon feedstock containing fromabout 0.2 to about 6.0 weight percent sulfur as organic sulfur compoundswherein (i) said feedstock is subjected to cracking in a reaction zonethrough contact with a particulate cracking catalyst at a temperature inthe range from 430° to 700° C.; (ii) cracking catalyst, which isdeactivated by sulfur-containing coke deposits, is separated fromreaction zone effluent and passes to a stripping zone wherein volatiledeposits are removed from said catalyst by contact with a stripping gascomprising steam at a temperature in the range from 430° to 700° C.;(iii) stripped catalyst is separated from stripping zone effluent andpasses to a catalyst regeneration zone and non-stripped,sulfur-containing coke deposits are removed from the stripped catalystby burning with an oxygen-containing regeneration gas at a temperaturein the range from 565° to 790° C., thereby forming sulfur oxides; and(iv) resulting catalyst is separated from regeneration zone effluent gasand recycled to the reaction zone; and wherein emissions of sulfuroxides in the regeneration zone effluent gas are reduced by the methodwhich comprises: (a) absorbing sulfur oxides in said regeneration zonewith fluidizable particulate solids which comprise at least oneinorganic oxide selected from the group consisting of the oxides ofaluminum, magnesium, zinc, titanium and calcium in association with atleast one free or combined rare earth metal selected from the groupconsisting of lanthanum, cerium, praseodymium, samarium and dysprosium,wherein said rare earth metal or metals and inorganic oxide or oxidesare present in the particulate solids in sufficient amount to effect theabsorption of at least about 50 weight percent of the sulfur oxidesproduced by the burning of sulfur-containing coke deposits in theregeneration zone and the ratio by weight of inorganic oxide or oxidesto rare earth metal or metals is from about 1.0 to about 30,000; and (b)removing said absorbed sulfur oxides from the fluidizing particulatesolids as a sulfur-containing gas which comprises hydrogen sulfide bycontacting said particulate solids with the hydrocarbon feedstock insaid reaction zone.

Another embodiment of the invention is a process for the cyclic,fluidized, catalytic cracking of a hydrocarbon feedstock containingorganic sulfur compounds wherein (i) said feedstock is subjected tocracking in a reaction zone through contact with a particulate crackingcatalyst at a temperature in the range from 430° to 700° C.; (ii)cracking catalyst, which is deactivated by sulfur-containing cokedeposits, is separated from reaction zone effluent and passes to astripping zone wherein volatile deposits are removed from said catalystby contact with a stripping gas comprising steam at a temperature in therange from 430 ° to 700° C.; (iii) stripped catalyst is separated fromstripping zone effluent and passes to a catalyst regeneration zone andnon-stripped, sulfur-containing coke deposits are removed from thestripped catalyst by burning with an oxygen-containing regeneration gasat a temperature in the range from 565° to 790° C., thereby formingsulfur oxides; and (iv) resulting catalyst is separated fromregeneration zone effluent gas and recycled to the reaction zone; andwherein emissions of sulfur oxides in the regeneration zone effluent gasare reduced by the method which comprises: (a) absorbing sulfur oxidesin said regeneration zone with a fluidizing particulate solid other thansaid cracking catalyst which comprises at least one inorganic oxideselected from the group consisting of the oxides of aluminum, magnesium,zinc, titantium and calcium in association with at least one free orcombined rare earth metal selected from the group consisting oflanthanum, cerium, praseodynium, samarium and dysprosium, wherein theratio by weight of inorganic oxide or oxides to rare earth metal ormetals is from about 1.0 to about 1,000 and said particulate solid isphysically admixed with said cracking catalyst; and (b) removing saidabsorbed sulfur oxides from the fluidizable particulate solid as asulfur-containing gas which comprises hydrogen sulfide by contactingsaid particulate solid with the hydrocarbon feedstock in said reactionzone.

In another embodiment, the present invention relates to a composition ofmatter prepared by the steps comprising: (a) impregnating a particulatesolid cracking catalyst comprising from about 0.5 to about 50 weightpercent of a crystalline aluminosilicate zeolite distributed throughouta matrix consisting essentially of from about 40 to about 100 weightpercent of alumina and from about 0 to about 60 weight percent of silicawith at least one rare earth metal compound selected from the groupconsisting of the compounds of lanthanum, cerium, praseodymium,samarium, and dysprosium, wherein the amount of said rare earth metalcompound or compounds is sufficient to add from about 0.004 to about 10weight percent of rare earth metal or metals, calculated as the metal ormetals, to said catalyst particles; and (b) calcining said impregnatedcatalyst particles at a temperature between about 200° and about 820° C.

In a further embodiment, the present invention relates to a compositionof matter comprising a particulate physical mixture of (a) a particulatesolid cracking catalyst for cracking hydrocarbons comprising acrystalline aluminosilicate zeolite distributed throughout a matrix; and(b) a particulate solid other than said cracking catalyst comprising atleast one inorganic oxide selected from the group consisting of theoxides of aluminum, magnesium, zinc, titanium and calcium in associationwith at least one free or combined rare earth metal selected from thegroup consisting of lanthanum, cerium, praseodymium, samarium anddysprosium, wherein the particulate solid other than cracking catalystcontains at least about 40 weight percent of the inorganic oxide oroxides, the ratio by weight of inorganic oxide or oxides to rare earthmetal or metals is from about 1.0 to about 1000, and said particulatesolid other than cracking catalyst comprises from about 0.1 to about 50weight percent of said particulate physical mixture.

In a still further embodiment, the present invention relates to acomposition of matter comprising a particulate physical mixture of (a) aparticulate solid cracking catalyst for cracking hydrocarbons comprisinga crystalline aluminosilicate zeolite distributed throughout a matrix,wherein said catalyst comprises from about 50 to about 99.9 weightpercent of the particulate physical mixture; (b) a first particulatesolid other than said cracking catalyst comprising at least about 50weight percent of one or more inorganic oxides selected from the groupconsisting of the oxides of aluminum, magnesium, zinc, titanium andcalcium; and (c) a second particulate solid other than said crackingcatalyst comprising at least one free or combined rare earth metalselected from the group consisting of lanthanum, cerium, praseodymium,samarium and dysprosium, wherein the ratio by weight of the inorganicoxide or oxides of said first particulate solid to the rare earth metalor metals of said second particulate solid is from about 1.0 to about1,000.

It has been discovered that the rare earth metal or metals and theinorganic oxide or oxides of this invention act together in asynergistic manner to afford a more efficient absorption of sulfuroxides from a gas than is possible if they are used separately.Accordingly, it is an object of this invention to provide an improvedcomposition of matter for use in absorbing sulfur oxides from a gas.

Another object of this invention is to provide an improved process forremoving sulfur oxides from a gas.

Another object of this invention is to provide an improved method forreducing sulfur oxide emissions from the regenerator of a cyclic,fluidized, catalytic cracking unit which does not significantly alterthe yield of hydrocarbon products from the cracking process.

A further object of this invention is to provide a highly active sulfuroxide absorbent for use in a cyclic, fluidized, catalytic cracking unitwhich does not significantly promote the combustion of carbon monoxidewithin the regeneration zone.

A still further object of this invention is to provide a highly activesulfur oxide absorbent for use in a cyclic, fluidized, catalyticcracking unit which does not significantly promote the formation ofnitrogen oxides within the regeneration zone.

Other objectives, aspects and advantages of the invention will bereadily apparent from the following detailed description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1-5 of the drawings illustrate the ability of 1% by weight ofvarious additives to improve the ability of CBZ-1 cracking catalyst toabsorb sulfur dioxide.

FIG. 6 of the drawings illustrates the ability of cerium to enhance theability of a high alumina cracking catalyst to absorb sulfur dioxidewhen deposited on the catalyst by impregnation as Ce(III) or Ce(IV).

DETAILED DESCRIPTION OF THE INVENTION

It has been discovered that an association of at least one free orcombined rare earth metal selected from the group consisting oflanthanum, cerium, praseodymium, samarium and dysprosium with at leastone inorganic oxide selected from the group consisting of the oxides ofaluminum, magnesium, zinc, titanium and calcium is a highly efficientabsorbent for sulfur oxides, which can be regenerated by contact with ahydrocarbon in the presence of a hydrocarbon cracking catalyst. The rareearth metal or metals and the inorganic oxide or oxides act together ina synergistic manner to absorb sulfur oxides from a gas more efficientlythan would be expected from their individual abilities to absorb sulfuroxides. Although the precise mechanism by which this synergism occurs isunclear, it is believed that the rare earth metal serves both to absorbthe sulfur oxides and to assist in the transfer of sulfur oxides to theinorganic oxide. This transfer of sulfur oxides to the inorganic oxidemay result from an ability of the rare earth metal to catalyze theconversion of sulfur dioxide to sulfur trioxide which is more easilyabsorbed by the inorganic oxide. The rare earth metal does not, however,significantly enhance the combustion of carbon monoxide which may alsobe present in the gas. Consequently, the process of this invention canbe used to reduce sulfur oxide emissions from the regenerator of a fluidcatalytic cracking unit wherein the regenerator vessel and/or associatedprocess equipment, such as cyclones, cannot tolerate the increasedtemperatures which would result from an enhanced combustion of carbonmonoxide. In addition, the rare earth metal, unlike metals such asplatinum and palladium, does not significantly enhance the formation ofnitrogen oxides in the regenerator of a fluidized catalytic crackingunit.

In the practice of this invention, at least one free or combined rareearth metal is employed which is selected from the group consisting oflanthanum, cerium, praseodymium, samarium and dysprosium. Preferably, atleast one free or combined metal selected from the group consisting oflanthanum and cerium is employed. Lanthanum and cerium are the mosteffective rare earth metals for sulfur oxide absorption in accordancewith this invention, and cerium is generally more active than lanthanum.

The rare earth metals are those elements which have atomic numbers from57 to 71. These elements commonly occur together in mineral deposits,and in those deposits which contain sufficient rare earth metals forcommercial exploitation, the total rare earth metal content generallyconsists of about 50% cerium, 20-30% lanthanum, 15-20% neodymium, 5-6%praseodymium and less than about 5% of the remaining rare earthelements. In view of the similar chemical properties of the various rareearth elements, it is relatively difficult to separate them or theircompounds from each other in pure form. For the practice of thisinvention, however, it is unneccessary to effect such a separation, anda preferred embodiment of the invention involves the use of a mixture ofrare earth metals or compounds thereof of the type which is obtainedfrom natural sources prior to any substantial separation of individualrare earth metals or compounds thereof. Additionally, it is relativelysimple to separate a cerium concentrate and a lanthanum concentrate frommixtures of rare earth metals, and these concentrates contain a majorportion of cerium and lanthanum respectively. A further embodiment ofthis invention involves the use of a mixture of rare earth metals orcompounds thereof such as is found in either of such concentrates.Cerium and lanthanum are highly preferred rare earth metals for use inthe practice of this invention, and the suitability of a cerium orlanthanum concentrate or a mixture of rare earth metals as initiallyobtained from natural deposits, is believed to be primarily a reflectionof the cerium and/or lanthanum content of these materials. Althoughpurified rare earth metals or compounds thereof are highly suitable foruse in the practice of this invention, such purification serves toincrease the cost of the necessary materials.

The oxides of the rare earth metals are particularly effective inassociation with the inorganic oxide or oxides of this invention forabsorbing sulfur oxides from a gas. Consequently, it is preferable toutilize the rare earth metal or metals in the form of the oxide oroxides. Although the rare earth metal oxides are preferred, it issufficient for the practica of this invention that at least one suitablerare earth metal or any compound thereof be employed.

In the practice of this invention, at least one inorganic oxide isemployed which is selected from the group consisting of the oxides ofaluminum, magnesium, zinc, titanium, and calcium, and preferably atleast one inorganic oxide selected from the group consisting of aluminaand magnesium oxide is employed. Magnesium oxide is generally somewhatmore efficient in absorbing sulfur oxides than is alumina but does notrelease absorbed sulfur oxides as readily as alumina upon contact with ahydrocarbon in the presence of a cracking catalyst. In addition, whenfluidized solids are employed, particles comprising large amounts ofmagnesium oxide frequently have poor attrition properties relative toparticles comprising large amounts of alumina. In view of the desirableattrition and sulfur oxide releasing properties of alumina, theinorganic oxide most preferably comprises alumina. Although the use ofany form of alumina is contemplated for use in the practice of thisinvention, gamma-alumina and eta-alumina are preferred because of theirusually large surface area.

A preferred embodiment of the invention involves the use of a mixture ofinorganic oxides which comprises at least about 50 percent by weight ofalumina. Mixtures of alumina with magnesium oxide and of alumina withzinc oxide are particularly suitable, wherein the weight ratio ofalumina to magnesium oxide or zinc oxide is desirably from about 1.0 toabout 500, and preferably from about 2.0 to about 100. In theseembodiments, the desirable attrition and sulfur oxide releasingproperties of alumina are combined with the excellent sulfur oxideabsorption properties of the other metal oxides, particularly ofmagnesium oxide or zinc oxide.

The inorganic oxides of this invention generally afford the best resultswhen they have a large surface area. This surface area is desirablygreater than about 10 square meters per gram, preferably greater thanabout 50 square meters per gram and ideally greater than about 100square meters per gram. Similarly, the rare earth metal or metalsgenerally afford the best results when they have a large surface areaexposed to the sulfur oxide containing gas as, for example, when therare earth metal or metals are deposited on a support having a largesurface area. Such a support will have a surface area which is desirablyin excess of about 10 square meters per gram, preferably greater thanabout 50 square meters per gram and ideally greater than about 100square meters per gram. The larger surface areas are most desirablebecause of a more efficient contacting of the sulfur oxide containinggas with the solid.

The ratio of inorganic oxide or oxides to rare earth metal or metals,calculated as the metal or metals, is desirably from about 0.1 to about30,000, more desirably from about 1.0 to about 30,000, preferably fromabout 1.0 to about 1,000, and more preferably from about 2.0 to about100, and ideally from about 3.0 to about 20. Decreasing the ratio ofinorganic oxide or oxides to rare earth metal or metals generallyaffords an improved sulfur oxide absorption until a ratio of about 3.0is reached. Smaller ratios than about 3.0 are not generally undesirable,but do not usually afford significant further improvement in sulfuroxide absorption properties. In addition, these smaller ratios requirelarger amounts of the rare earth metal or metals which will generally bemore expensive than the inorganic oxide or oxides.

Suitable hydrocarbon cracking catalysts for use in the practice of thisinvention include all high-activity solid catalysts which are stableunder the required conditions. Suitable catalysts include those of theamorphous silica-alumina type having an alumina content of about 10 toabout 30 weight percent. Catalysts of the silica-magnesium type are alsosuitable which have a mangesia content of about 20 weight percent.Preferred catalysts include those of the zeolite-type which comprisefrom about 0.5 to about 50 weight percent and preferably from about 1 toabout 30 weight percent of a crystalline aluminosilicate componentdistributed throughout a porous matrix. Zeolite type cracking catalystsare preferred because of their thermal stability and high catalyticactivity.

The crystalline aluminosilicate or zeolite component of the zeolite-typecracking catalyst can be of any type or combination of types, natural orsynthetic, which is known to be useful in catalyzing the cracking ofhydrocarbons. Suitable zeolites include both naturally occurring andsynthetic aluminosilicate materials such as faujasite, chabazite,mordenite, Zeolite X (U.S. Pat. No. 2,882,244), Zeolite Y (U.S. Pat. No.3,130,007) and ultrastable large-pore zeolites (U.S. Pat. Nos. 3,293,192and 3,449,070). The crystalline aluminosilicates having a faujasite-typecrystal structure are particularly suitable and include naturalfaujasite, Zeolite X and Zeolite Y. These zeolites are usually preparedor occur naturally in the sodium form. The presence of this sodium isundesirable, however, since the sodium zeolites have a low stabilityunder hydrocarbon cracking conditions. Consequently, for use in thisinvention the sodium content of the zeolite is ordinarily reduced to thesmallest possible value, generally less than about 1.0 weight percentand preferably below about 0.3 weight percent through ion exchange withhydrogen ions, hydrogen-precursors such as ammonium ion, or polyvalentmetal cations including calcium, magnesium, strontium, barium and therare earth metals such as cerium, lanthanum, neodymium and theirmixtures. Suitable zeolites are able to maintain their pore structureunder the high temperature conditions of catalyst manufacture,hydrocarbon processing and catalyst regeneration. These materials have auniform pore structure of exceedingly small size, the cross-sectiondiameter of the pores being in the range from about 4 to about 20angstroms, preferably from about 8 to about 15 angstroms.

The matrix of the zeolite-type cracking catalyst is a porous refractorymaterial within which the zeolite component is dispersed. Suitablematrix materials can be either synthetic or naturally occurring andinclude, but are not limited to, silica, alumina, magnesia, boria,bauxite, titania, natural and treated clays, kieselguhr, diatomaceousearth, kaolin and mullite. Mixtures of two or more of these materialsare also suitable. Particularly suitable matrix materials comprisemixtures of silica and alumina, mixtures of silica with alumina andmagnesia, and also mixtures of silica and alumina in combination withnatural clays and clay-like materials. Mixtures of silica and aluminaare preferred, however, and contain preferably from about 10 to about 65weight percent of alumina mixed with from about 35 to about 90 weightpercent of silica, and more preferably from about 25 to about 65 weightpercent of alumina mixed with from about 35 to about 75 weight percentof silica.

In the practice of this invention, the rare earth metal or metals, whichare associated with one or more suitable inorganic oxides, arepreferably used in a form which does not involve chemical incorporationwithin a zeolite. Consequently, the rare earth metal or metals of thisinvention for use in the absorption of sulfur oxides are preferably notincorporated into a zeolite, for example by ion-exchange techniques, andare in addition to any such rare earth metal or metals which may be soincorporated in a zeolite. Such ion-exchanged rare earth metal or metalsare not detrimental to the practice of this invention, but this form ofrare earth metal is relatively inactive with respect to the absorptionof sulfur oxides.

In the practice of this invention, the rare earth metal or metals andinorganic oxide or oxides can be combined in any suitable manner and canbe additionally combined with the hydrocarbon cracking catalyst. Thesematerials, for example, can be combined and shaped into pellets orextrudates of any desired shape. In a highly preferred embodiment, therare earth metal or metals, the inorganic oxide or oxides, and thehydrocarbon cracking catalyst are employed in the form of particulatefluidizable solids. In this embodiment, the particles should besufficiently strong that they are not subject to excessive attrition anddegradation during fluidization. The average size of the solid particleswill be desirably in the range from about 20 microns or less to about150 microns, and preferably less than about 50 microns. The use offluidized solids provides a highly efficient technique for contacting agas with a solid or solids as is required in the practice of the processof this invention. Consequently, the use of fluidized solids affords avery efficient method of contacting the rare earth metal or metals andinorganic oxide or oxides of this invention with a gas which containssulfur oxides. Similarly, the use of fluidized solids also provides anefficient method of contacting the spent rare earth metal-inorganicoxide absorbent of this invention with a hydrocarbon in the presence ofa cracking catalyst to remove the absorbed sulfur oxides.

When particulate solids are used in the practice of this invention, theparticles of cracking catalyst can contain both the rare earth metal ormetals and inorganic oxide or oxides. Alternatively, the particles ofcracking catalyst can contain the rare earth metal or metals and bephysically mixed with a separate particulate solid which comprises theinorganic oxide or oxides. As a further alternative, the particles ofcracking catalyst can contain the inorganic oxide or oxides and bephysically mixed with a separate particulate solid which comprises therare earth metal or metals. In addition, a physical mixture of threedifferent particulate solids can also be employed wherein oneparticulate solid comprises the cracking catalyst, the rare earth metalor metals comprise the second particulate solid, and the inorganic oxideor oxides comprise the third particulate solid. It will, of course, beappreciated that combinations of these four different alternatives arealso possible.

The inorganic oxide or oxides of this invention can comprise a portionof a cracking catalyst as, for example, in the case of a silica-aluminaor silica-magnesia catalyst. Also, the inorganic oxide or oxides of thisinvention can comprise at least a portion of the matrix of azeolite-type cracking catalyst. A particularly preferred embodiment ofthis invention comprises the use of alumina as the inorganic oxide whichis provided in the form of a zeolite-type cracking catalyst havingalumina in its matrix. The alumina content of such a matrix is desirablyfrom about 10 to about 100 weight percent, preferably from about 40 toabout 100 weight percent, more preferably from about 60 to about 100weight percent, and ideally from about 70 to about 100 weight percent.The use of a zeolite-type cracking catalyst having a high aluminamatrix, for example in excess of about 40 weight percent, provides ahighly convenient manner in which to provide the inorganic oxide of thisinvention. As the alumina content of the matrix increases, the abilityof the cracking catalyst to absorb sulfur oxides in accordance with thisinvention also increases.

The inorganic oxide or oxides and/or rare earth metal or metals of thisinvention can be in the form of a fluidizable powder which is admixedwith a particulate cracking catalyst. Illustrative of such powders arealumina, magnesia, titania, zinc oxide, calcium oxide, cerium oxide,lanthanum oxide and mixed rare earth oxides comprising cerium and/orlanthanum.

The rare earth metal or metals of this invention can be incorporatedinto or deposited onto a suitable support. Suitable supports include,but are not limited to, amorphous cracking catalyst, zeolite-typecracking catalysts, silica, alumina, mixtures of silica and alumina,magnesia, mixtures of silica and magnesia, mixtures of alumina andmagnesia, mixtures of alumina and magnesia with silica, titania, zincoxide, calcium oxide, natural and treated clays, kieselguhr,diatomaceous earth, kaolin and mullite. Such support preferablycomprises at least one of the inorganic oxides of this invention.Desriably, the support is porous and has a surface area, including thearea of the pores open to the surface, of at least about 10, preferablyat least about 50, and most preferably at least about 100 square metersper gram. Large surface areas are desirable because of a more efficientcontacting of the sulfur oxide containing gas with the solid.

Similarly, the inorganic oxide or oxides of this invention can beincorporated into or deposited onto a suitable support. This supportshould also be porous and desirably has a surface area of at least about10, preferably at least about 50, and most preferably at least about 100square meters per gram. Large surface areas are desirable because of amore efficient contacting of the sulfur oxide containing gas with thesolid. Suitable supports include, but are not limited to, silica,natural and treated clays, kieselguhr, diatomaceous earth, kaolin andmullite. In addition, one of the inorganic oxides of this invention, forexample alumina, can be used as a support for one or more otherinorganic oxides.

The rare earth metal or metals and/or inorganic oxide or oxides of thisinvention can be combined with a support either during or afterpreparation of the support. One method consists of impregnating asuitable support with an aqueous or organic solution or dispersion of acompound or compounds of the rare earth metal or metals and/or metal ormetals of the inorganic oxide or oxides. The impregnation can be carriedout in any manner which will not destroy the structure of the support.After drying, the composite can be calcined to afford the supported rareearth metal or metals and/or inorganic oxide or oxides of the invention.Preferably, nitrates, carbonates and salts of organic acids such asacetates are employed in the impregnating solution or dispersion,particularly if a cracking catalyst is used as the support, since theresidue from the thermal decomposition of these salts is relativelyinnocuous to the activity of a hydrocarbon cracking catalyst. Thehalogen and sulfate salts can also be used, but the byproducts producedduring thermal degradation of these salts may be deleterious to theactivity of the cracking catalyst. Consequently, the halogen and sulfatesalts are used, preferably, in combination with supports which aresubstantially inert to the cracking of hydrocarbons.

The rare earth metal or metals and/or inorganic oxide or oxides can beincorporated with a support precursor, such as silica gel,silica-alumina gel or alumina gel, prior to spray drying or otherphysical formation process. Subsequent drying and, if desired,calcination then affords the supported rare earth metal or metals and/orinorganic oxide or oxides. In those instances wherein a crackingcatalyst is employed as a support, the rare earth metal or metals and/orthe inorganic oxide or oxides may be incorporated by coprecipitation ofthe rare earth metal or metals and/or metal or metals of the inorganicoxide or oxides with catalyst precursors, for example as the metalhydroxides, followed by addition of the zeolite component if any inparticulate form, followed by drying and, it desired, calcination.

The gas from which sulfur oxides are removed according to the process ofthe present invention can contain, in addition to sulfur oxides, suchgases as nitorgen, steam, carbon dioxide, carbon monoxide, oxygen,nitrogen oxides and rare gases such as argon. Minor amounts of othergases may also be present. Suitable sulfur oxide containing gasesinclude, but are not limited to, flue gases, tail gases and stack gases.Ordinarily, gases such as nitrogen, carbon dioxide and steam willrepresent a major portion of the gas. The process of this invention isparticularly effective for removing sulfur oxides from a gas which has alow concentration of sulfur oxides, for example, less than about 0.5volume percent. The process of this invention is also effective,however, for removing sulfur oxides from a gas which has a highconcentration of sulfur oxides, for example, greater than about 0.5volume percent and up to about 10 volume percent. The process of thisinvention permits removal from the sulfur oxide containing gas ofdesirably at least about 50%, preferably at least 80% and ideally morethan about 90% of the sulfur oxides.

The gas from which sulfur oxides are removed according to the process ofthis invention need not contain molecular oxygen, but in a preferredembodiment desirably contains an amount of molecular oxygen which is inexcess of the stoichiometric amount required to convert any sulfurdioxide present to sulfur trioxide. The excess of oxygen can range fromabout 0.001 to about 10,000 times the stoichiometric amount which isrequired to convert any sulfur dioxide to sulfur trioxide. Ordinarily,however, the excess need not be greater than from about 0.001 to about100 times the stoichiometrically required amount. The excess ofmolecular oxygen need not be large, but the ability of the rare earthmetal-inorganic oxide absorbent of this invention to absorb sulfurdioxide is improved as the amount of excess molecular oxygen increases.Although the reason for this effect by molecular oxygen is uncertain, itis believed that increased concentrations of oxygen promote theconversion of sulfur dioxide to sulfur trioxide in accordance with thelaw of mass action. It is further believed that this sulfur trioxide ismore easily absorbed by the rare earth metal-inorganic oxide absorbentthan is the surfur dioxide. The molecular oxygen can either beinherently present in the sulfur oxide containing gas or can be addedthereto.

The absorption of sulfur oxides with the rare earth metal-inorganicoxide absorbent of this invention is desirably carried out at atemperature below about 900° C., preferable at a temperature from about100° to about 900° C. and most preferably at a temperature from about300° to about 800° C.

The removal of absorbed sulfur oxides from the rare earthmetal-inorganic oxide absorbent of this invention is accomplished bycontacting the spent absorbent with a hydrocarbon in the presence of ahydrocarbon cracking catalyst at an elevated temperature. Thistemperature is desirably from about 375° to about 900° C., preferablyfrom about 430° to about 700° C. and most preferably from about 450° toabout 650° C.

The temperature at which the sulfur oxides are abosrbed by and removedfrom the rare earth metal-inorganic oxide absorbent must, of course, belower than that which will cause substantial thermal deactivation of thecracking catalyst. Consequently, acid treated clays cannot ordinarily beused at temperatures much above about 650° C., whereas many zeolite-typecracking catalysts can be used at temperatures of 750° C. and above. Byway of example, hydrocarbon cracking catalysts containing ultrastablezeolites are stable at temperatures in excess of 1000° C.

Any hydrocarbon can be used to remove the absorbed sulfur oxides fromthe rare earth metal-inorganic oxide of this invention so long as it canbe cracked by the cracking catalyst at the temperatures employed.Suitable hydrocarbons include, but are not limited to, methane, naturalgas, natural gas liquids, naphtha, light gas oils, heavy gas oils,wide-cut gas oils, vacuum gas oils, decanted oils and reduced crude oilsas well as hydrocarbon fractions derived from shale oils, coalliquefaction and the like. Such hydrocarbons can be employed eithersingly or in any desired combination.

In a preferred embodiment of the invention, the rare earthmetal-inorganic oxide absorbent is contacted with added steam while itis simultaneously contacted with a hydrocarbon in the presence of thehydrocarbon cracking catalyst. In an alternative embodiment, the rareearth metal-inorganic oxide absorbent is contacted with steam at atemperature desirably from about 100° to about 900° C. and preferablyfrom about 430° to about 700° C. subsequent to the treatment with ahydrocarbon in the presence of a hydrocarbon cracking catalyst. Suchtreatment with steam is not necessary, but generally results in animproved removal of absorbed sulfur oxides. The amount of steam employedis desirably equal to or greater on a mole basis than the amount ofsulfur oxides absorbed by the rare earth metal-inorganic oxideabsorbent. The amount of added steam can range, on a mole basis, fromabout 1.0 to about 10,000, preferably from about 1.0 to about 1,000, andmore preferably from about 1.0 to about 100 times the amount of sulfuroxides absorbed by the absorbent.

Although the invention disclosed herein is not to be so limited, it isbelieved that chemical reaction occurs between the rare earthmetal-inorganic oxide absorbent and the sulfur oxides which results inthe formation of nonvolatile inorganic sulfur compounds, such assulfites and sulfates. This chemical reaction is reversible and can besummarized in a simplified manner by the following equations:

    M.sub.x O+SO.sub.2 →M.sub.x SO.sub.3

    M.sub.x O+SO.sub.3 →M.sub.x SO.sub.4

where x is the ratio of the oxidation state of the oxide ion to theoxidation state of a metal component M of the rare earth metal-inorganicoxide absorbent when combined with oxygen. At very high temperatures,these sulfites and sulfates can undergo partial decomposition toliberate the original sulfur oxides and absorbent. As a consequence ofthis reversal of the sulfur oxide absorption at high temerature, theabsorption of sulfur oxides is desirably effected at a temperature belowabout 900° C. and preferably below about 800° C.

The precise mechanism by which absorbed sulfur oxides are removed fromthe rare earth metal-inorganic oxide absorbent of this invention isunknown, but it is believed that the combination of hydrocarbon andhydrocarbon cracking catalyst at elevated temperatures provides areducing environment which effects a conversion of absorbed sulfuroxides to hydrogen sulfide while simultaneously reactivating theabsorbent for further absorption of sulfur oxides. Although theinvention is not to be so limited, it is believed that the removal ofabsorbed sulfur oxides can be summarized in a simplified manner by thefollowing equations:

    M.sub.x SO.sub.3 +3H.sub.2 →M.sub.x O+H.sub.2 S+2H.sub.2 O(a)

    M.sub.x SO.sub.4 +4H.sub.2 →M.sub.x O+H.sub.2 S+3H.sub.2 O(b)

    M.sub.x SO.sub.3 +3H.sub.2 →M.sub.x S+3H.sub.2 O→M.sub.x O+H.sub.2 S+2H.sub.2 O                                    (c)

    M.sub.x SO.sub.4 +4H.sub.2 →M.sub.x S+4H.sub.2 O→M.sub.x O+H.sub.2 S+3H.sub.2 O                                    (d)

where x is the ratio of the oxidation state of the oxide ion to theoxidation state of a metal component M of the rare earth metal-inorganicoxide absorbent when combined with oxygen. The removal of absorbedsulfur oxides from the absorbent is generally improved by contacting theabsorbent with added steam either simultaneously with or subsequent totreatment with a hydrocarbon in the presence of a cracking catalyst. Itis believed that at least some metal sulfide is formed according toequations (c) and (d) above and that added steam serves to promote theconversion of these metal sulfides to hydrogen sulfide with simultaneousreactivation of the absorbent.

The hydrogen sulfide which is produced during the removal of absorbedsulfur oxides from the rare earth metal-inorganic oxide absorbent can beconverted to elemental sulfur by any of the conventional techniqueswhich are well known to the art as, for example, in a Claus unit.Cracked hydrocarbon products which are produced during removal ofabsorbed sulfur oxides from the absorbent of this invention, aftersubstantial separation of hydrogen sulfide, can be recycled toextinction for further use in removing absorbed sulfur oxides.Alternatively, these cracked hydrocarbon products can be burned directlyas a fuel or can be fractionated by conventional techniques to separatemore valuable products of lower molecular weight than the initialhydrocarbon employed.

A highly preferred embodiment of this invention comprises its use toreduce sulfur oxide emissions from catalyst regeneration in a cyclic,fluidized, catalytic cracking process. In this embodiment, the rareearth metal-inorganic oxide absorbent is circulated through thefluidized catalytic cracking process in association with the particulatecracking catalyst.

Catalytic cracking of heavy mineral oil fractions is one of the majorrefining operations employed in the conversion of crude oils todesirable fuel products such as high-octane gasoline fuels used inspark-ignited internal combustion engines. In fluidized catalyticcracking processes, high molecular weight hydrocarbon liquids or vaporsare contacted with hot, finely-divided, solid catalytic particles,either in a fluidized bed reactor or in an elongated riser reactor, andthe catalyst-hydrocarbon mixture is maintained at an elevatedtemperature in a fluidized or dispersed state for a period of timesufficient to effect the desired degree of cracking to low molecularweight hydrocarbons of the kind typically present in motor gasoline anddistillate fuels.

Conversion of a selected hydrocarbon feedstock in a fluidized catalyticcracking process is effected by contact with a cracking catalyst,preferably in one or more fluidized transfer line reactors, atconversion temperature and at a fluidizing velocity which limits theconversion time to not more than about ten seconds. Conversiontemperatures are desirably in the range from about 430° to about 700° C.and preferably from about 450° to about 650° C. Reactor effluent,comprising hydrocarbon vapors and cracking catalyst containing adeactivating quantity of carbonaceous material or coke, is thentransferred to a separation zone. Hydrocarbon vapors are then separatedfrom spent cracking catalyst and the catalyst stripped of volatiledeposits before regeneration. The stripping zone can be suitablymaintained at a temperature in the range from about 430° to about 700°C., preferably from about 450° to about 650° C., and most preferablyfrom about 465° to about 595° C. The preferred stripping gas is steam,although inert gases, such as nitrogen or flue gases, or mixtures ofsteam with inert gases can also be used. The stripping gas is introducedat a pressure in the range from about 0.7 to about 2.5 kilograms persquare centimeter above atmospheric pressure, and in an amount which issufficient to effect substantially complete removal of volatile depositsfrom deactivated cracking catalyst. When steam is employed as thestripping gas, the weight ratio of stripping steam to cracking catalystis in the range from about 0.0005 to about 0.025 and preferably in therange from about 0.0015 to about 0.0125.

In the catalytic cracking of hydrocarbons, some nonvolatile carbonaceousmaterial or coke is deposited on the catalyst particles. Coke compriseshighly condensed aromatic hydrocarbons which generally contain a minoramount of hydrogen, generally from about 4 to about 10 weight percent ofhydrogen. When the hydrocarbon feedstock contains organic sulfurcompounds, the coke also contains sulfur. As coke builds up on thecracking catalyst, the activity of the catalyst for cracking and theselectivity of the catalyst for producing gasoline blending stocksdiminishes. The catalyst can, however, recover a major portion of itsoriginal capabilities by removal of most of the coke therefrom in asuitable regeneration process.

In a fluidized catalytic cracking process, stripped deactivated crackingcatalyst is regenerated by burning the coke deposits from the catalystsurface with a molecular oxygen containing regeneration gas, such asair, in a regeneration zone or regenerator. This burning results in theformation of combustion products such as sulfur oxides, carbon monoxide,carbon dioxide and steam. The oxygen containing regeneration gas cancontain diluent gases such as nitrogen, steam, carbon dioxide, recycledregenerator effluent and the like. The molecular oxygen concentration ofthe regeneratioon gas is ordinarily from about 2 to about 30 volumepercent and preferably from about 5 to about 25 volume percent. Sinceair is conveniently employed as a source of molecular oxygen, a majorportion of the inert gas can be nitrogen. The regenerator zonetemperatures are ordinarily in the range from about 565° to about 790°C. and are preferably in the range from about 620° to about 735° C. Whenair is used as the regeneration gas, it enters the bottom of theregenerator from a blower or compressor and a fluidizing velocity in therange from about 0.05 to about 1.5 meters per second and preferably fromabout 0.15 to about 0.90 meters per second is maintained in theregenerator. Regenerated catalyst is then recycled to the transfer linereactor for further use in the conversion of hydrocarbon feedstock.

The method of this invention can be used in a fluidizing catalyticcracking process with wide variation in the cracking conditions. In theusual case where a gas oil feedstock is employed, the throughput ratio(TPR), or volume ratio of total feed to fresh feed, can vary from about0.1 to about 3.0. Conversion level can vary from about 40% to about 100%where conversion is here defined as the percentage reduction ofhydrocarbons boiling above 221° C. at atmospheric pressure by formationof lighter materials or coke. The weight ratio of catalyst to oil in thereactor can vary within the range from about 2 to about 20 so that thefluidized dispersion will have a density in the range from about 15 toabout 320 kilograms per cubic meter. Fluidizing velocity may be in therange from about 3.0 to about 30 meters per second. This crackingprocess is preferably effected in a vertical transfer line reactorwherein the ratio of length to average diameter is at least about 25.

A suitable hydrocarbon feedstock for use in a fluidized catalyticcracking process in accordance with this invention can contain fromabout 0.2 to about 6.0 weight percent of sulfur in the form of organicsulfur compounds. Advantageously, the feedstock contains from about 0.5to about 5 weight percent sulfur and more advantageously contains fromabout 1 to about 4 weight percent sulfur wherein the sulfur is presentin the form of organic sulfur compounds. Suitable feedstocks include,but are not limited to, sulfur-containing petroleum fractions such aslight gas oils, heavy gas oils, wide-cut gas oils, vacuum gas oils,naphthas, decanted oils, residual fractions and cycle oils derived fromany of these as well as sulfur-containing hydrocarbon fractions derivedfrom shale oils, tar sands processing, synthetic oils, coal liquefactionand the like. Any of these suitable feedstocks can be employed eithersingly or in any desired combination.

With respect to the effective use of this invention in a fluidizedcatalytic cracking process, the stripped deactivated cracking catalystin association with the rare earth metal-inorganic oxide absorbent isregenerated in the regeneration zone and the sulfur oxides produced bycombustion of the sulfur-containing coke are absorbed by the absorbent.The hydrocarbon feedstock is then cracked in the presence of theregenerated cracking catalyst in association with the rare earthmetal-inorganic oxide absorbent containing absorbed sulfur oxides.During the catalytic conversion of the hydrocarbon feedstock, theabsorbed sulfur oxides are substantially released from the absorbent asa sulfur-containing gas comprising hydrogen sulfide. The deactivatedcracking catalyst in association with the rare earth metal-inorganicoxide absorbent is then stripped with a steam containing stripping gasprior to recycle to the regeneration zone. This steam stripping servesnot only to remove volatile hydrocarbon deposits, but also serves tocomplete the removal of any residual absorbed sulfur oxides from theabsorbent as a sulfur-containing gas which comprises hydrogen sulfideand completes the reactivation of the absorbent for further absorptionof sulfur oxides in the regeneration zone. The resulting hydrogensulfide is recovered together with the other volatile products from thereaction and stripping zones and is separated and can be converted toelemental sulfur in facilities which are conventionally associated witha fluidized catalytic cracking unit.

When the process of this invention is employed in a fluidized catalyticcracking process, the regeneration zone effluent gases desirably containat least about 0.01 volume percent, preferably at least about 0.5 volumepercent, more preferably at least about 1.0 volume percent and ideallyat least about 2.0 volume percent of molecular oxygen. In addition, thecombination of rare earth metal or metals and inorganic oxide or oxidesis preferably used in sufficient amount to effect the absorption of atleast about 50%, more preferably at least about 80% and ideally morethan about 90% of the sulfur oxides produced in the regeneration zone bythe combustion of coke. As a result, the concentration of sulfur oxidesin the regeneration zone effluent gas stream can be maintained at lessthan about 600 parts per million by volume (ppmv), advantageously lessthan about 200 ppmv and more advantageously at less than about 100 ppmv.In conventional fluidized catalytic cracking processes which do notemploy the process of this invention, the cracking of high-sulfurfeedstocks often results in the formation of a regeneration zoneeffluent gas stream which contains 1200 ppmv or more of sulfur oxides.

This invention is highly suitable for use in reducing emissions ofsulfur oxides from the regenerator of a fluidized catalytic crackingunit since the rare earth metals and inorganic oxides of the inventionhave little or no adverse effect on the yield of desirable low molecularweight hydrocarbon products from hydrocarbon cracking.

With further reference to the use of this invention to reduceregeneration zone sulfur oxide emissions in a fluidized catalyticcracking process, the rare earth metal or metals and/or inorganic oxideor oxides can be deposited on a suitable support by introducing one ormore compounds of the desired rare earth metal or metals and/or one ormore precursors of the inorganic oxide or oxides into the fluidizedcatalytic cracking process cycle and thereby depositing the rare earthmetal or metals and/or inorganic oxide or oxides onto the support insitu. In this embodiment, the support will comprise cracking catalyst.The rare earth metal compound or inorganic oxide precursor can beintroduced as an aqueous or organic solution or dispersion, or in thesolid, liquid or gaseous state at any stage of the cracking processcycle which comprises the cracking reaction zone, the stripping zone andthe regeneration zone. For example, such compound or precursor can beadmixed either with the feedstock or fluidizing gas in the reactionzone, with the regeneration gas, torch oil or water in the regenerationzone, or with the stripping gas in the striping zone, or can beintroduced as a separate stream. Suitable compounds or precursors for insitu incorporation include, but are not limited to, inorganic metalsalts such as nitrates and carbonates, organometallic compounds, metaldiketonates, and metal carboxylates of from 1 to 20 carbon atoms.

A particularly suitable embodiment of the invention for use in afluidized catalytic cracking process involves the circulation throughthe process cylce in admixture with the cracking catalyst of aparticulate solid other than cracking catalyst which comprises at leastone inorganic oxide selected from the group consisting of the oxides ofaluminum, magnesium, zinc, titanium and calcium in association with atleast one free or combined rare earth metal selected from the groupconsisting of lanthanum, cerium, praseodymium, samarium and dysprosium;wherein the ratio by weight of inorganic oxide or oxides to rare earthmetal or metals is preferably from about 1.0 to about 1,000, morepreferably from about 2.0 to about 100, and most preferably from about3.0 to about 20; and wherein the particulate solid other than crackingcatalyst preferably contains at least about 40 weight percent and morepreferably at least about 60 weight percent of the inorganic oxide oroxides. In a particularly preferred version of this embodiment, theparticles other than cracking catalyst comprise a fluidizable highsurface area particulate alumina upon or into which the rare earth metalor metals are deposited or incorporated. The particulate compositionformed by mixing the cracking catalyst and particulate solid other thancracking catalyst comprises an amount of cracking cayalyst which isdesirably from about 50 to about 99.9 weight percent, preferably fromabout 70 to about 99.5 weight percent, and more preferably from about 90to about 99.5 weight percent based on the total mixture. Conversely, thecomposition comprises an amount of particulate solid other than crackingcatalyst which is desirably from about 0.1 to about 50 weight percent,preferably from about 0.5 to about 30 weight percent, and morepreferably from about 0.5 to about 10 weight percent based on the totalmixture.

Another embodiment of the invention for use in a fluidized catalyticcracking process involves the circulation through the process cycle inadmixture with the particulate cracking catalyst of (a) a firstparticulate solid other than cracking catalyst which comprises at leastabout 50 weight percent of one or more inorganic oxides selected fromthe group consisting of the oxides of aluminum, magnesium, zinc,titanium and calcium and (b) a second particulate solid other thancracking catalyst which comprises at least one free or combined rareearth metal selected from the group consisting of lanthanum, cerium,praseodymium, samarium and dysprosium. In this embodiment, the ratio byweight of the inorganic oxide or oxides of the first particulate solidto the rare earth metal or metals of the second particulate solid ispreferably from about 1.0 to about 1,000 and more preferably from about2.0 to about 100. In addition, the composition formed by mixing thecracking catalyst and particulate solids other than cracking catalystcomprises an amount of cracking catalyst which is desirably from about50 to about 99.9 weight percent, preferably from about 70 to about 99.5weight percent, and most preferably from about 90 to about 99.5 weightpercent based on the total mixture. Particulate cerium oxide, lanthanumoxide and mixtures of rare earth oxides comprising cerium and/orlanthanum are highly suitable for use as the second particulate solidother than cracking catalyst. Particulate alumina, especiallygamma-alumina, and particulate solids comprising alumina are highlysuitable for use as the first particulate solid other than crackingcatalyst.

Another particularly suitable embodiment of the invention for use in afluidized catalytic cracking process involves the use of a crackingcatalyst in the process which is prepared by the steps comprising (a)impregnating a particulate solid cracking catalyst comprising from about0.5 to about 50 weight percent of a crystalline alumicosilicate zeolitedistributed throughout a porous matrix comprised of from about 40 toabout 100 weight percent of alumina and from about 0 to about 60 weightpercent of silica with a least one rare earth metal compound selectedfrom the group consisting of the compounds of lathanum, cerium,praseodymium, samarium and dysprosium, wherein the amount of said rareearth metal compound or compounds is sufficient to add from about 0.004to about 10 weight percent rare earth metal or metals, calculated as themetal or metals, to the particles of cracking catalyst and (b) calciningthe impregnated catalyst particles of a temperature between about 200°and about 820° C. Preferably, the catalyst matrix has a high aluminacontent and comprises in excess of about 50 weight percent, morepreferably in excess of about 60 weight percent, and ideally in excessof about 70 weight percent of alumina. The particulate cracking catalystis preferably impregnated with sufficient rare earth metal compound orcompounds to add from about 0.1 to about 5 weight percent rare earthmetal or metals, calculated as the metal or metals, to said catalystparticles. Cerium and lathanum compounds are preferred for use inimpregnating the cracking catalyst. Unexpectedly, the use of cericcompounds for impregnation affords a composition which is far moreactive for the absorption of sulfur oxides than does the use of cerouscompounds. The rare earth metal or metals added by impregnation are inaddition to any rare earth metal or metals which may be present in thecrystalline aluminosilicate zeolite as a consequence of ion exchangewith rare earth metals.

The following examples are intended only to illustrate the invention andare not to be construed as imposing limitations on the invention.

EMAMPLE 1

A particulate alpha alumina monohydrate (CATAPAL-SB, obtained from theConoco Chemicals Division of Continental Oil Company) analyzing for74.2% Al₂ O₃, 0.008% SiO₂, 0.005% Fe₂ O₃, 0.004% Na₂ O₃, and less than0.01% sulfur, having a buld density in the range from 660 to 740 gramsper liter, and with 78% by weight of the particles having a size lessthan 90 microns, was dried at 220° C. for 3 hours and then calcined at650° C. for 3 hours to afford a particulate gamma-alumia having a waterporosity of 1.32 grams of water per gram. Nine hundred grams of thisgamma-alumina was impregnated with an aqueous solution prepared bydissolving 391.3 grams of ceric ammonium nitrate [Ce(NH₄)₂ (NO₃)₆ ] insufficient distilled water at room temperature to give 1200 millilitersof solution. The impregnated gamma-alumina was then dried at 120° C.overnight and calcined at 650° C. for 3 hours to give a particulategamma-alumina having 10.0 weight percent of cerium deposited thereupon,with an attrition rate of 6.31% and a particle size distribution asfollows: 23% greater than 80 microns, 25.2% from 20 to 40 microns and3.8% fines.

EXAMPLE 2

A particulate alpha alumina monohydrate (CATAPAL-SB, obtained from theConoco Chemicals Division of Continental Oil Company) having theproperties described in Example 1 was calcined at 540° C. for 3 hours toproduce a particulate gamma-alumina. A solution of 7.1 grams of cerousnitrate [Ce(NO₃)₃.6H₂ O] in 25 milliliters of water was then used toimpregnate 20.6 grams of the particlate gamma-alumina. The impregnatedalumina was then dried at 120° C. and calcined at 540° C. for 3 hours togive a particulate alumina which contained 10.0 weight percent ofcerium.

EXAMPLE 3

The procedure of Example 2 was repeated, except that a solution of 8.3grams of lanthanum nitrate [La(NO₃)₃.5H₂ O] in 30 milliliters of waterwas used to impregnate 25.0 grams of the particulate gamma-alumina. Theimpregnated alumina was dried and calcined as in Example 2 to afford aparticulate alumina which contained 10.0 weight percent of lanthanum.

EXAMPLE 4

The procedure of Example 2 was repeated, except that a solution of 8.5grams of neodymium nitrate [Nd(NO₃)₃.6H₂ O] in 30 milliliters of waterwas used to impregnate 25.3 grams of the particulate gamma-alumina. Theimpregnated alumina was dried and calcined as in Example 2 to afford aparticulate alumina which contained 10.0 weight percent of neodymium.

EXAMPLE 5

The procedure of Example 2 was repeated, except that a solution of 7.6grams of dysprosium nitrate [Dy(NO₃)₃.5H₂ O] in 30 milliliters of waterwas used to impregnate 25.2 grams of the particulate gamma-alumina. Theimpregnated alumina was dried and calcined as in Example 2 to afford aparticulate alumina which contained 10.0 weight percent of dysprosium.

EXAMPLE 6

The procedure of Example 2 was repeated, except that a solution of 8.7grams of praseodymium nitrate [Pr(NO₃)₃.6H₂ O] in 30 milliliters ofwater was used to impregnate 25.3 grams of the particulategamma-alumina. The impregnated alumina was dried and calcined as inExample 2 to afford a particulate alumina which contained 10.0 weightpercent of praseodymium.

EXAMPLE 7

The procedure of Example 2 was repeated, except that 25.1 grams of theparticulate gamma-alumina was impregnated with a solution prepared bydissolving 3.3 grams of mixed rare earth oxides (48% cerium, 20%lanthanum, 13% neodymium, 5% praseodymium and 14% others) in 10milliliters of concentrated nitric acid followed by diluting with 15milliliters of water and 5 milliliters of 30% hydrogen peroxide. Enoughadditional water was then added to just cover the alumina. Theimpregnated alumina was dried and calcined as in Example 2 to afford aparticulate alumina which contained 10 weight percent of mixed rareearth metals.

EXAMPLE 8

The procedure of Example 2 was repeated, except that a solution of 8.2grams of samarium nitrate [Sm(NO₃)₃.6H₂ O] in 30 milliliters of waterwas used to impregnate 25.1 grams of the particulate gamma-alumina. Theimpregnated alumina was dried and calcined as in Example 2 to afford aparticulate alumina which contained 10.0 weight percent of samarium.

EXAMPLE 9

The procedure of Example 2 was repeated, except that solution of 8.1grams of gadolinium nitrate [Gd(NO₃)₃.6H₂ O] in 30 milliliters of waterwas used to impregnate 25.3 grams of the particulate gamma-alumina. Theimpregnated alumina was dried and calcined as in Example 2 to afford aparticulate alumina which contained 10.0 weight percent of gadolinium.

EXAMPLE 10

A mixture of 50 grams of magnesium oxide, 40 grams of alpha aluminamonohydrate and 10 grams of calcium oxide was kneaded with 100milliliters of 10% nitric acid. The resulting mixture was dried at 120°C., calcined first at 540° C. for 3 hours and finally calcined at 1200°C. for an additional 3 hours to afford a particulate solid comprised ofMgO, Ca₃ Al₁₀ O₁₈ and MgAl₂ O₄. A solution of 3.9 grams of cericammonium nitrate [Ce(NH₄)₂ (NO₃)₆ ] in about 15 milliliters of methanolwas then used to impregnate 9.0 grams of the particulate solid. Excessmethanol was evaporated and the impregnated solid then dried at 120° C.and calcined at 540° C. for 3 hours to give a particulate solidcontaining magnesium, aluminum and calcium oxides which also contained10.0 weight percent of cerium.

EXAMPLE 11

A silica-alumina composite (obtained from American Cyanamide Co.) wasdried at 120° C., calcined at 650° C. for 3 hours, ground, and passedthrough a 100 mesh sieve to afford a particulate solid analyzing for70.0% SiO₂ and 20.7% Al₂ O₃ which had a surface area of 516 squaremeters per gram. A solution of 11.0 grams of ceric ammonium nitrate[Ce(NH₄)₂ (NO₃)₆ ] in water was then used to impregnate 25.2 grams ofthe particulate silica-alumina. The impregnated silica-alumina was driedat 120° C. and calcined at 540° C. for 3 hours to give particulatesilica-alumina which contained 10.0 weight percent of cerium.

EXAMPLE 12

A solution of 22.2 grams of zinc nitrate [Zn(NO₃)₂.6H₂ O] in 500milliliters of water was mixed with 542 grams of an alumina hydrosol(analyzing for 9.5% Al₂ O₃ and obtained from American Cyanamide Co.) ina Waring blender and gelled by addition of 25 milliliters ofconcentrated ammonium hydroxide solution. The gel was dried overnight at120° C., calcined at 540° C. and ground to pass through a 100 meshsieve. A solution of 11.3 grams of ceric ammonium nitrate [Ce(NH₄)₂(NO₃)₆ ] in 20 milliliters of water was then used to impregnate 25.9grams of the particulate solid. The impregnated solid was dried at 120°C. and calcined 540° C. for 3 hours to give a particulate solidcontaining aluminum and zinc oxides which also contained 10.0 weightpercent of cerium.

EXAMPLE 13

A solution of 4.35 grams of ceric ammonium nitrate [Ce(NH₄)₂ (NO₃)₆ ] inabout 10 milliliters of water was used to impregnate 10.0 grams ofparticulate rutile (TiO₂). The impregnated rutile was dried at 120° C.and calcined at 540° C. for 3 hours to give a particulate titaniumdioxide which also contained 10.0 weight percent of cerium.

EXAMPLE 14

A particulate silica (Grade 62, obtained from Davison Chemical Division,W. R. Grace & Co.) having a pore volume of 1.15 milliliters per gram, asurface area of 340 square meters per gram, a bulk density of 400kilograms per cubic meter, and a nominal 60-200 mesh size was dried at120° C., calcined at 540° C. for 3 hours, and ground to pass through a100 mesh sieve. A solution of 11.0 grams of cerci ammonium nitrate[Ce(NH₄)₂)NO₃)₆ ] in water was then used to impregnate 25.2 grams of thecalcined and ground silica. The impregnated silica was dried at 120° C.and calcined at 540° C. for 3 hours to give a particulate silica whichcontained 10.0 weight percent of cerium.

EXAMPLE 15

A solution of 4.35 grams of ceric ammonium nitrate [Ce(NH₄)₂ (NO₃)₆ ] inabout 10 milliliters of water was used to impregnate 10.0 grams ofparticulate anatase (TiO₂). The impregnated anatase was dried at 120° C.and calcined at 540° C. for 3 hours to give a particulate titaniumdioxide which also contained 10.0 weight percent of cerium.

EXAMPLE 16

A solution of 11.2 grams of ceric ammonium nitrate [Ce(NH₄)₂ (NO₃)₆ ] in25 milliliters of water was used to impregnate 25.7 grams of equilibriumHEZ-55 particulate cracking catalyst (Houdry Division of Air Productsand Chemicals, Inc.) analyzing for 60% Al₂ O₃ and 35% SiO₂, andcontaining a rare earth ion exchanged Y-type zeolite in a silica-aluminamatrix. The impregnated cracking catalyst was dried at 120° C. andcalcined at 540° C. for 3 hours to give a particulate cracking catalystwhich contained 10.0 weight percent of cerium deposited thereupon.

EXAMPLE 17

Commercially available particulate low surface area alpha-alumina wasimpregnated with an aqueous solution of magnesium nitrate in foursuccessive steps to give a particulate solid containing 10 to 14 weightpercent of magnesium. In a fifth step, the solid was impregnated withsufficient ceric ammonium nitrate [Ce(NH₄)₂ (NO₃)₆ ] in methanol togive, after drying and calcination at 540° C., a particulate aluminaupon which magnesium oxide (10-14 weight percent magnesium) and 10%cerium were deposited.

EXAMPLE 18

A solution of 0.0093 grams of cerous nitrate [Ce(NO₃)₃.6H₂ O] in 20milliliters of water was used to impregnate 20.0 grams of HFZ-20particulate cracking catalyst (Houdry Division of Air Products andChemicals, Inc.) analyzing for 59.4% Al₂ O₃, 36.1% SiO₂ and 0.97% Na₂ O,having a surface area of about 390 square meters per gram and containingabout 20 to about 25 weight percent hydrogen Y-zeolite. The impregnatedcracking catalyst was dried at 120° C. and calcined at 540° C. for 3hours to give an HFZ-20 catalyst having 150 parts per million by weightof cerium deposited on its surface by way of a Ce(III) salt.

EXAMPLE 19

A 25.0 gram sample of the HFZ-20 cracking catalyst described in Example18 was impregnated with a solution of 0.0195 grams of ceric ammoniumnitrate [Ce(NH₄)₂ (NO₃)₆ ] in 22.5milliliters of water. The impregnatedcracking catalyst was dried at 120° C. and calcined at 540° C. for 3hours to give an HFZ-20 catalyst having 148 parts per million by weightof cerium deposited on its surface by way of a Ce(IV) salt.

EXAMPLE 20

The ability of various compositions to absorb sulfur dioxide from a gasstream was measured using the following procedure. A 1.00 gram sample ofthe composition was placed on top of a plug of glass wool in a quartzsample tube having a diameter of 1.3 centimeters and a length of 41centimeters. The sample tube was then placed in a tube furnace andheated to the desired temperature while a purge gas composed of about2.6 volume percent water vapor in helium was passed downward through thesample bed at a flow rate of 10 cubic centimeters per minute. After a 1hour purge, a synthetic gas composed of 0.10 volume percent sulfurdioxide, 2.9 volume percent oxygen, 2.6 volume percent water vapor andthe remaimder being helium, was passed downward through the sample bedat a rate of 10 cubic centimeters per minute. After passage through thesample bed, the gas stream was periodically sampled and the samplesanalyzed with a gas chromatograph for sulfur dioxide content. uponcompletion of a test, and without altering the gas flow rate, the testsample was removed from the sample tube. After a purge of 15 to 20minutes, the effluent gas from the empty sample tube was sampled at thesame intervals as for the test sample and analyzed in the same manner.The amount of sulfur dioxide absorbed by the test sample was thencalculated by comparing the amount of sulfur dioxide in the effluentfrom the test sample at a given time with the amount of sulfur dioxidefrom the empty sample tube.

Test samples were prepared by mixing 1 part by weight of an additivewith 99 parts by weight of CBZ-1 particulate cracking catalyst (DavisonChemical Division, W. R. Grace & Co.) analyzing for 29.1, Al₂ O₃, 0.46%Na₂ O and 0.11% Fe, and containing a Y-zeolite. Both the additive andthe CBZ-1 cracking catalyst were steamed at 760° C. (100% steam atatmospheric pressure) for 5 hours prior to mixing. The various additivesare set forth in Table 1, and the test results are set forth in FIG. 1-5as indicated in Table 1. The test samples were all evaluated at 675° C.

FIG. 1-5 plot the ability of each test sample to absorb sulfur dioxidefrom the gas stream as a function of time, and the area under each curveis a measure of the total amount of sulfur dioxide absorbed. FIG. 1-4(Run No. 1 of Table 1) demonstrate that CBZ-1 catalyst alone is a poorsulfur oxide absorbent. FIG. 2 (Run No. 8 of Table 1) demonstrates thatthe addition of 1% by weight of gamma-alumina to CBZ-b 1 catalyst causesvery little improvement in its ability to absorb sulfur dioxide.Similarly, FIG. 5 (Run No. 15 of Table 1) demonstrates that 1% of anadditive composed of cerium on silica has little effect on the abilityof CBZ-1 catalyst to absorb sulfur dioxide. FIG. 1 (Run No. 4 ofTable 1) and FIG. 3 (Run No. 10 of Table 1) demonstrate that neodymiumand gadolinium in association with alumina do not significantly improvethe ability of CBZ-1 catalyst to absorb sulfur dioxide, and theseresults show that not all of the rare earth metals are satisfactory foruse in the practice of this invention. The remaining results which areset forth in FIG. 1-5 demonstrate the marked ability or rare earthmetals selected from the group consisting of lanthanum, cerium,praseodymium, samarium, dysprosium, and mixtures thereof when inassociation with one or more inorganic oxides selected from the groupconsisting of the oxides of aluminum, magnesium, zinc, titanium andcalcium to improve the absorption of sulfur dioxide by CBZ-1 catalyst.

EXAMPLE 21

The ability of CBZ-1 particulate cracking catalyst (Davison ChemicalDivision, W. R. Grace & Co.) alone and in association with cerium and/oralumina to absorb sulfur dioxide at 750° C. was measured using the testprocedure described in Example 20. In each case, the absorptionexperiments were carried out for a test period of 92 minutes. Except forparticulate CeO₂, the additives, CBZ-2 and cerium modified CBZ-1 wereseparately steamed at 760° C. (100% steam at atmospheric pressure) for 5hours prior to use. The results, which are set forth in Table 2,demonstrate that synergistically improved results are obtained bycombining cerium with alumina. The combination of cerium with aluminaresults in a substantially greater absorption of sulfur dioxide thanwould be predicted on the basis of their individual abilities to absorbsulfur dioxide.

                  TABLE 1                                                         ______________________________________                                        Run   Additive        Additive     Test                                       No.   Composition     Preparation  Results                                    ______________________________________                                        1     None (pure CBZ-1                                                                              --           FIGS. 1-4                                        employed)                                                               2     l0.0% Ce on alumina                                                                           Example 2    FIG. 1                                     3     10.0% La on alumina                                                                           Example 3    FIG. 1                                     4     10.0% Nd on alumina                                                                           Example 4    FIG. 1                                     5     10.0% Dy on alumina                                                                           Example 5    FIG. 1                                     6     10.0% Pr on alumina                                                                           Example 6    FIG. 2                                     7     10.0% mixed rare                                                                              Example 7    FIG. 2                                           earth metals on alumina                                                 8     gamma-alumina   Unimpregnated                                                                              FIG. 2                                                           alumina of                                                                    Example 2                                               9     10.0% Sm on alumina                                                                           Example 8    FIG. 3                                     10    10.0% Gd on alumina                                                                           Example 9    FIG. 3                                     11    10.0% Ce on     Example 10   FIG. 4                                           MgO--CaO--Al.sub.2 O.sub.3                                              12    10.0% Ce on rutile                                                                            Example 11   FIG. 4                                           (TiO.sub.2)                                                             13    10.0% Ce on     Example 12   FIG. 4                                           ZnO--Al.sub.2 O.sub.3                                                   14    10.0% Ce on anatase                                                                           Example 15   FIG. 5                                           (TiO.sub.2)                                                             15    10.0% Ce on SiO.sub.2                                                                         Example 14   FIG. 5                                     ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                                   Amount of                                          Run                        SO.sub.2 Absorbed,                                 No.   Test Sample          microliters.sup.(a)                                ______________________________________                                        1     CBZ-cracking catalyst                                                                              385, 436                                           2     5 parts particulate gamma-                                                                         439                                                      Al.sub.2 O.sub.3.sup.(b) mixed with 95 parts                                  CBZ-1 cracking catalyst                                                 3     25 ppm Ce on CBZ-1 cracking                                                                        429                                                      catalyst.sup.(c)                                                        4     CBZ-1 cracking catalyst mixed                                                                      339, 350                                                 with 192 ppm of particulate                                                   CeO.sub.2                                                               5     5 parts of particulate gamma-                                                                      512                                                      Al.sub.2 O.sub.3 mixed with 95 parts of                                       25 ppm Ce on CBZ-1 cracking                                                   catalyst.sup.(c)                                                        6     5 parts of 500 ppm Ce on gamma-                                                                    515                                                      Al.sub.2 O.sub.3.sup.(d) mixed with 95 parts of                               CBZ-1 cracking catalyst                                                 7     CBZ-1 cracking catalyst mixed                                                                      522                                                      with 192 ppm of particulate                                                   CeO.sub.2 and 5% gamma Al.sub.2 O.sub.3.sup.b                           ______________________________________                                         .sup.(a) One microliter at 20° C. is equal to 3.33 ×             10.sup.-6 grams of SO.sub.2.                                                  .sup.(b) Unimpregnated gammaalumina of Example 2.                             .sup.(c) Cerium was added to the CBZ1 catalyst by impregnation with an        aqueous solution of ceric ammonium nitrate followed by drying at              120° C. and calcination at 540° C. for 3 hours.                 .sup.(d) Cerium was added to the unimpregnated gamma alumina of Example 2     by impregnation with an aqueous solution of ceric ammonium nitrate            followed by drying at 120° C. and calcination at 540° C. fo     3 hours.                                                                 

EXAMPLE 22

The ability of HFZ-20 cracking catalyst to absorb sulfur dioxide, bothbefore and after impregnation with about 150 ppm of cerium, was measuredat 657° C. using the test procedure described in Example 20. Theimpregnated catalysts of Examples 18 and 19 were emploted, which wereprepared by impregnation of CBZ-1 catalyst with a Ce(III) salt and aCe(IV) salt respectively. These results are set forth in FIG. 6 andserve to indicate that the use of a Ce(IV) salt affords a more activematerial than does the use of a Ce(III) salt.

EXAMPLE 23

The effect on the catayltic activity of CBZ-1 cracking catalyst (DavisonChemical Division, W. R. Grace & Co.) of the unimpregnated gamma-aluminaof Example 1 and the 10.0% cerium on alumina prepared according toExample 1 was determined by measurement of the relative microactivity(RMA) and coke factor of CBZ-1 catalyst and of various mixtures of CBZ-1catalyst with the alumina or cerium on a alumina. The CBZ-1 catalyst andthe additives were steamed at 760° C. (100% steam at atmosphericpressure) for 5 hours prior to use. The RMA is the ratio of thecatalytic activity of a cracking catalyst to a standard crackingcatalyst at constant severity and serves as a measure of the crackingactivity of a catalyst. The coke factor is a measure of the tendency ofa cracking catalyst to convert a hydrocarbon feedstock to coke, with alarge coke factor indicating a tendency toward higher coke formation.The coke factor serves as a measure of the selectivity of a crackingcatalyst. The results of these measurements are set forth in Table 3,and serve to demonstrate that the addition of particulate gamma-aluminato CBZ-1 cracking catalyst causes a significant reduction in itsactivity and selectivity. In distinct contrast, addition of the 10%cerium on alumina additive to CBZ-1 catalyst does not reduce

                  TABLE 3                                                         ______________________________________                                        Run                                                                           No.   Catalyst Composition.sup.(a)                                                                    RMA      Coke Factor                                  ______________________________________                                        1     CBZ-1             170.4    1.27                                                                 173      1.30                                         2     5 parts Al.sub.2 O.sub.3 mixed with                                                             161.9    1.33                                               95 parts CBZ-1                                                          3     15 parts Al.sub.2 O.sub.3 mixed with                                                            153.6    1.47                                               85 parts CBZ-1                                                          4     2 parts 10.0% Ce on Al.sub.2 O.sub.3                                                            179      1.14                                               mixed with 98 parts CBZ-1                                               5     5 parts 10.0% Ce on Al.sub.2 O.sub.3                                                            179      1.16                                               mixed with 95 parts CBZ-1                                               6     10 parts 10.0% Ce on Al.sub.2 O.sub.3                                                           165      1.31                                               mixed with 90 parts CBZ-1                                               7     15 parts 10.0% Ce on Al.sub.2 O.sub.3                                                           181      1.12                                               mixed with 85 parts CBZ-1                                               ______________________________________                                         .sup.(a) Compositions are expressed in terms of parts by weight.   the        activity or selectivity of the catalyst and, indeed, effects a slight     improvement in both RMA and coke factor.

EXAMPLE 24

Pilot plant cyclic fluidized catalytic cracking tests were conductedwith a wide boiling gas oil feedstock having a sulfur content of 1.33weight percent and a nitrogen content of 0.0841 weight percent. percent.Comparison test were carried out using equilibrium CBZ-1 particulatecracking catalyst (Davison Chemical Divison, W. R. Grace & Co. ) andalso using a mixture of 99 parts by weight of equilibrium CBZ-1 with 1part by weight of the 10.0% cerium on alumina additive preparedaccording to Example 2. In addition, comparison tests were also carriedout using equilibrium HFZ-33 particulate cracking catalyst (HoudryDivision of Art Products and Chemicals, Inc.) analyzing for 59% Al₂ O₃and 37% SiO₂ and also using mixture of 99 parts by weight of equilibrimHFZ-33 catalyst with 1 part by weight of the 10.0% cerium on aluminaadditive prepared according to Example 2. In each case, the cerium onalumina additive was steamed at 760° C. (100% steam at atmosphericpressure) for 5 hours prior to use. The cerium on alumina additivereduced sulfur oxide emissions in the regenerator flue gas from 894 to397 parts per million by volume or 56% for CBZ-1 catalyst and from 270to 129 parts per million by volume or 52% for HFZ-33 catalyst. Thecomparative data are set forth in Table 4.

EXAMPLE 25

Pilot plant cyclic fluidized catalytic cracking tests were conductedwith a gas oil feedstock having a sulfur content of 2.50 weight percentand a nitrogen content of 0.102 weight percent. Comparison tests werecarried out using equilibrium CBZ-1 particulate cracking catalyst(Davison Chemical Division, W. R. Grace & Co.) and also using a mixtureof 99 parts by weight of equilibrium CBZ-1 catalyst with 1 part byweight of the

                  TABLE 4                                                         ______________________________________                                                    Test                                                                                    B               D                                                             CBZ-1           HFZ-33                                                A       plus 1%  C      plus 1%                                 Catalyst:     CBZ-1   additive HFZ-33 additive                                ______________________________________                                         Cracking Conditions:                                                         Reactor Temp.,                                                                              510     510      510    510                                     °C.                                                                    Feed Rate,    9       9        9      9                                       g./min.                                                                       Stripping Conditions:                                                         Temp., °C.                                                                           496     496      496    496                                     Steam, g./hr. 16      16       16     16                                      Nitrogen, std.                                                                              0.031   0.031    0.031  0.031                                   cu.m./hr.                                                                     Regeneration                                                                  Conditions:                                                                   Temp., °C.                                                                           649     649      649    649                                     Flue Gas O.sub.2                                                                            2.0     2.0      2.0    2.0                                     Conc., Vol. %                                                                 Flue Gas SO.sub.2 Conc.,                                                                    894     397      270    129                                     ppmv.sup.(a)                                                                  Flue Gas Nitrogen                                                                           8       6        34     29                                      Oxide Conc., ppmv                                                             ______________________________________                                         .sup.(a) Net sulfur emitted to the atmosphere.   10.0% cerium on alumina      additive prepared according to Example 2. The cerium on alumina additive     was steamed at 760° C. (100% steam at atmospheric pressure for 5     hours prior to use. The comparative data are set forth in Table 5.     Conversion and product yields were essentially the same in both cases.

                  TABLE 5                                                         ______________________________________                                                         Test                                                                                    B                                                                     A       CBZ-1 plus                                         Catalyst:          CBZ-1   1% additive                                        ______________________________________                                         Cracking Conditions:                                                         Feed rate, g./min. 13      13                                                 Reactor Temp., ° C.                                                                       510     511                                                Catalyst to Oil wt. ratio                                                                        5.8     5.1                                                WHSV               17.6    17.2                                               Stripping Conditions:                                                         Temp., °C.  496     496                                                Nitrogen, std. cu.m./hr.                                                                         0.031   0.031                                              Regeneration Conditions:                                                      Temp., °C.  621     621                                                Carbon on Regenerated                                                                            0.082   0.068                                              Catalyst, wt. %                                                               Flue Gas O.sub.2 Conc., Vol. %                                                                   2.0     2.0                                                Products:                                                                     Conversion, vol. % 70.63   69.72                                              Yield (wt. %)                                                                 H.sub.2 S          0.69    0.70                                               C.sub.2 and lighter                                                                              1.51    1.58                                               C.sub.3            5.23    5.31                                               C.sub.4            9.40    9.39                                               C.sub.5 -221° C.                                                                          47.22   45.93                                              221° C. +   31.75   32.65                                              Coke               4.19    4.44                                               ______________________________________                                    

We claim:
 1. A process for removing sulfur oxides from a gas whichcomprises:(a) absorbing sulfur oxides from the gas with an absorbentwhich comprises an inorganic oxide composition in association with atleast one free or combined rare earth metal selected from the groupconsisting of lanthanum, cerium, praseodymium, samarium and dysprosiumat a temperature in the range from about 100° to about 900° C., whereinsaid inorganic oxide composition comprises at least one member selectedfrom the group consisting of MgAl₂ O₄, and mixtures of alumina withmagnesium oxide and the ratio by weight of inorganic oxide compositionto rare earth metal or metals is from about 1.0 to about 1,000; and (b)removing absorbed sulfur oxides from the absorbent as a sulfurcontaining gas by contacting said absorbent with a hydrocarbon in thepresence of a hydrocarbon cracking catalyst at a temperature in therange from about 375° to about 900° C.
 2. The process as set forth inclaim 1 wherein said inorganic oxide composition comprises a mixture ofalumina with magnesium oxide and the weight ratio of alumina tomagnesium oxide is from about 1.0 to about
 500. 3. The process as setforth in claim 1 wherein said inorganic oxide composition comprises amixture of magnesium oxide with MgAl₂ O₄.
 4. The process as set forth inclaim 1 wherein said inorganic oxide composition comprises MgAl₂ O₄. 5.The process as set forth in claim 1 wherein said free or combined rareearth metal is selected from the group consisting of lanthanum andcerium.
 6. The process as set forth in claim 2 wherein said free orcombined rare earth metal comprises on oxide selected from the groupconsisting of the oxides of lathanum and cerium.
 7. The process as setforth in claim 1 wherein said free or combined rare earth metalcomprises cerium and said cerium is in the form of a cerium-containingmixture of free or combined rare earth metals wherein cerium is themajor component of said mixture of rare earth metals.
 8. The process asset forth in claim 1 wherein the ratio by weight of inorganic oxidecomposition to rare earth metal or metals is from about 2.0 to about100.
 9. The process as set forth in claim 1 wherein the sulfur oxidecontaining gas also contains an amount of molecular oxygen which is inexcess of the amount required to convert any sulfur dioxide present tosulfur trioxide.
 10. The process as set forth in claim 1 wherein theabsorbent containing absorbed sulfur oxides is simultaneously contactedwith added steam while it is contacted with said hydrocarbon, and theamount of steam is greater, on a mole basis, than the amount of sulfuroxides absorbed by the absorbent.
 11. The process as set forth in claim1 wherein the absorbent is contacted with steam subsequent to saidcontacting with a hydrocarbon, and the amount of steam is greater, on amole basis, than the amount of sulfur oxides absorbed by the absorbent.12. A process for the cyclic. fluidized catalytic cracking of ahydrocaborn feedstock containing organic sulfur compounds wherein: (i)said feedstock is subjected to cracking in a reaction zone throughcontact with a particulate cracking catalyst at a temperature in therange from 430° to 700° C.; (ii) cracking catalyst, which is deactivatedby sulfur-containing coke deposits, is separated from reaction zoneeffluent and passes to a stripping zone wherein volatile deposits areremoved from said catalyst by contact with a stripping gas comprisingsteam at a temperature in the range from 430° the 700° C.; (iii)stripped catalyst is separated from stripping zone effluent and passesto a catalyst regeneration zone and nonstripped, sulfur-containing cokedeposits are removed from the stripped catalyst by burning with anoxygen-containing regeneration gas at a temperature in the range from565° to 790° C., thereby forming sulfur oxides; and (iv) resultingcatalyst is separated from regeneration zone effluent gas and recycledto the reaction zone; and wherein emission of sulfur oxides in theregeneration zone effluent gas are reduced by the method whichcomprises:(a) absorbing sulfur oxides in said regeneration zone withfluidizable particulate solids which comprise and inorganic oxidecomposition in association with at least one free or combined rare earthmetal selected from the group consisting of lanthanum, cerium,praseodymium, samarium and dsprosium, wherein said inorganic oxidecomposition and rare earth metal or metals are present in theparticulate solids in sufficient amount to affect the absorption of atleast 50 weight percent of the sulfur oxides produced by the burning ofsulfur-containing coke deposits in the regeneration zone, said inorganicoxide composition comprises at least one member selected from the groupconsisting of MgAl₂ O₄, and mixtures of alumina with magnesium oxide andthe ratio by weight of inorganic oxide composition to rare earth metalor metals is from about 1.0 to about 1,000; and (b) removing absorbedsulfur oxides from the fluidizable particulate solids as asulfur-containing gas by contacting said particulate solids with thehydrocarbon feedstock in said reaction zone.
 13. The process as setforth in claim 12 wherein the rare earth metal or metals and inorganicoxide composition are contaimed within the particles of crackingcatalyst.
 14. The process as set forth in claim 13 wherein said rareearth metal or metals are in non-ion-exchanged form.
 15. A process forthe cyclic, fluidized catalytic cracking of a hydrocabon feedstockcontaining organic sulfur compounds wherein: (i) said feedstock issubjected to cracking in reaction zone through contact with aparticulate cracking catalyst at a temperature in the range from 430° to700° C.; (ii) cracking catalyst, which is deactivated bysulfur-containing coke deposits, is separated from reaction zoneeffluent and passes to a stripping zone wherein volatile deposits areremoved from said catalyst by contact with a stripping gas comprisingsteam at a temperature in the range from 430° to 700° C.; (iii) strippedcatalyst is separated from stripping zone effluent and passes to acatalyst regeneration zone and nonstripped, sulfur-containing cokedeposits are removed from the stripped catalyst by burning with anoxygen-containing regeneration gas at a temperature in the range from565° to 790° C., thereby forming sulfur oxides; and (iv) resultingcatalyst is separated from regeneration zone effluent gas and recycledto the reaction zone; and wherein emissions of sulfur oxides in theregeneration zone effluent gas are reduced by the method whichcomprises:(a) absorbing sulfur oxides in said regeneration zone with afluidizable particulate solid other than said cracking catalyst whichcomprises an inorganic oxide composition in association with at leastone free or combined rare earth metal selected from the group consistingof lanthanum, cerium, praseodymium, samarium and dysprosium, whereinsaid inorganic oxide composition comprises at least one member selectedfrom the group consisting of MgAl₂ O₄, and mixtures of alumina withmagnesium oxide the ratio by weight of inorganic oxide composition torare earth metal or metals is from about 1.0 to about 1,000 and saidparticulate solid is physically admixed with said cracking catalyst; and(b) removing absorbed sulfur oxides from the fluidizable particulatesolid as a sulfur-containing gas by contacting said particulate solidwith the hydrocarbon feedstock in said reaction zone.
 16. The process asset forth in claim 15 wherein the regeneration zone effluent gascontains at least about 0.5 volume percent of molecular oxygen.
 17. Theprocess as set forth in claim 15 wherein the amount of said fluidizableparticulate solid other than cracking catalyst is from about 0.5 toabout 10 weight percent of the total mixture of cracking catalyst andparticulate solid other than cracking catalyst.
 18. The process as setforth in claim 15 wherein said inorganic oxide composition comprises amixture of alumina with magnesium oxide and the weight ratio of aluminato with magnesium oxide is from about 1.0 to about
 500. 19. The processas set forth in claim 15 wherein said inorganic oxide compositioncomprises a mixture of magnesium oxide with MgAl₂ O₄.
 20. The process asset forth in claim 15 wherein said inorganic oxide composition comprisesMgAl₂ O₄.